1. Field of the Invention
This invention is in the field of heat dissipating devices for use with electric motors.
2. Description of the Prior Art
The present invention relates in general to resistor units for the dynamic braking of large electric motors such as those used to power locomotives and specifically to a resistor grid design which reduces the heat transferred from the resistance material to the insulator mounting material.
During the braking of electric motors, the coasting motor continues to generate a large amount of excess electrical energy that must be dissipated. It has been customary in the art to divert such excess energy into banks of resistor grids. Such resistor grids have typically been arranged in the form of a plurality of parallel metal grids, with the grids being affixed to and positioned between a pair of insulator members. During braking, the high current values shunted into the resistor grids elevate grid temperatures to such a point that melting occurs unless the grids can be effectively cooled. Thus, it is known that it is necessary to preserve uniform air spaces between the parallel grids so that heat produced in the grids may be readily dissipated by convection to the surrounding air. Typically, a blower is used to aid the heat convection process. Furthermore, the stresses due to repeated expansion and contraction of the heated resistance grid members each time braking occurs must be taken into account to avoid failure of the grid members. Unless the heat expansion of the grids takes place in a controlled manner, the grids may warp out of their parallel alignment, thus resulting in a non-uniform cooling air flow and consequent unit failure.
In attempting to solve the aforementioned problems, various resistor grid structures have been devised as shown in U.S. Pat. Nos. 2,721,920 to Weide; 2,772,337 to DuBois; 2,858,402 to Griffes et al; 2,874,257 to Kuhn et al, and 3,543,213 to Weyenberg. Such grid structures have proven to be overly complex, therefore requiring expensive manufacturing and assembly procedures. For example, U.S. Pat. No. 2,721,920 issued to Weide teaches the use of a continuous one-piece formed grid supported between insulator members by means of brackets. Costly manufacturing equipment is required to fabricate such a one-piece grid and, once formed, such a grid is highly subject to damage during the assembly process. Further, the forming machines for such a one-piece grid structure are capable of making grids of but a single width and gauge thickness thus reducing unit component flexibility. The continuous grid structure also has the disadvantage of expanding in both vertical and horizontal directions upon heating. If the horizontal expansion is not precisely uniform throughout the structure, a warping of the parallel grids is caused thus reducing cooling air flow to the grid sections resulting in heat damage to the grids and consequent unit failure. U.S. Pat. No. 2,772,337 teaches the welding of resistance grids to circular pins which are then inserted into insulator members. The welding of planar grids to each side of a circular pin has proven to be costly and time consuming from a manufacturing aspect and, further, renders the structure susceptible to vibrational failures in a locomotive environment. U.S. Pat. No. 2,858,402 requires spreader plates along the length of the grid assemblies to prevent heat expansion warping of the parallel grids. In addition, the latter patent discloses welded grid portions extending entirely through open slots in the insulator members requiring costly means for mounting the insulator members to the outer casing. U.S. Pat. No. 2,874,257 uses a continuous one-piece grid structure and thus encounters problems similar to those of the Weide patent. U.S. Pat. No. 3,543,213 teaches the use of separate insulator blocks at each weld joint of a series of U-shaped grids strips. The grid strips extend through the separate insulator blocks and the construction is such that the grid strips must bend upon heat expansion. Thus, a complex assembly is required using a multiple insulator block construction. Unless the grid heat expansion bending is precisely uniform, a disadvantageous distribution of cooling air will result.
In Patent Application Ser. No. 892,887 filed Apr. 3, 1978, now abandoned, I suggested positioning welded grid panels between two slotted insulator members with a clearance between the grids and one insulator member to allow heat expansion of the grids solely in a one direction during operation. This structure addressed the resistor cooling and expansion problems, as well as the complexity and expense problems associated with the other improvements developed in the art.
Recent governmental restrictions on the use of asbestos as an insulating material have created a need for a resistor grid design which will allow the use of lower temperature insulating materials. The heat from the current carrying resistor grid is transferred to the insulator by conduction, due to the method of mounting the grid to the insulator. Heat transfer by radiation and convection also occurs due to the proximity of the current carrying portion of the resistor grid to the insulator and the fact that forced air is used to aid in transferring heat away from the grid.
Improvements in resistor grid structures have helped solve problems associated with grid warping and cooling, and have made the manufacturing of grid structures easier and less expensive. The present invention is addressed to continuing to solve the aforementioned problems, and further allowing for the use of lower temperature insulating materials in resistor grid structures.